1. Field of the Invention
The present invention relates to a forming method and a forming device of a fracture start portion required when stretching and fracturing a ductile metal part such as a connecting rod, for example, as well as a fracturing method and a fracturing device of a ductile metal part.
2. Description of the Related Art
A halved part obtained by stretching and fracturing a ductile metal part is combined again to be used as a pair of products in some cases. An example of such a part is a connecting rod known as a part for vehicles. And a method of fracturing a connecting rod by applying a tensile stress on a large end of the connecting rod among the large end and a small end provided respectively at both ends of the connecting rod has been known. In such a fracturing method, a technique to form a groove of a large width and a small depth at a fracture portion through machining has been used for a long time (See, for example, U.S. Pat. No. 5,208,979 (pp. 1-3, FIG. 3).).
In the meantime, instead of a method of forming a connecting rod fracture portion using such machining, a method of forming a stress riser groove of smaller width and larger depth to be a trigger of connecting rod fracture using laser is also known (See, for example, U.S. Pat. No. 5,208,979 (pp. 1-3, FIG. 3).).
Also, in relation to formation of such a stress riser groove with laser, a method of forming a so-called fracture groove by continuously forming a recess part and a rib formed between the recess parts through use of the laser is also known (See, for example, U.S. Pat. No. 5,882,438 (pp. 5-6, FIG. 3).).
In the above methods of forming a groove for fracturing a connecting rod in a patent document 1 and a patent document 2, since laser is irradiated onto an inner circumferential surface of a connecting-rod large end with a fine pulse so as to form a so-called groove for fracture, influence of heat generated at a (specific) irradiation position of the laser is excessively transmitted to an adjoining position of the groove, and there is a fear that this extra heat might cause fusion more than necessary at a portion adjacent to the groove.
Also, by such a phenomenon, even though a groove of a predetermined width should be formed normally, the groove might be formed with an uneven groove width.
Therefore, when such a groove not in the desired shape is formed as a connecting-rod fracture start portion, a clean connecting-rod fracture surface can not be obtained in some cases.